Abstract: Fuzzy synthetic evaluation method can be problematic when it is applied to groundwater quality assessment due to absolutization of membership degree or mismatch between national groundwater quality standard and classification criteria required by fuzzy synthetic evaluation. Here, we created an optimized fuzzy synthetic evaluation model to solve these problems. We introduced the concept of relative membership degree for establishing a dynamic fuzzy relationship between evaluation index and groundwater quality standard. Furthermore, we used the optimized model to analyze and assess the groundwater quality of the Dagu River Basin in Qingdao for validation. The results show that the national groundwater quality standard satisfied the requirements of evaluation criteria type in the optimized model. The optimized model solved the incompatibility problem in the traditional model. When the measure index fell in a mid-grade interval, the relative membership degrees for this and two adjacent grades were greater than 0. Compared with the absolute distribution of membership degree, the optimized model expanded the distribution over different grades and reflected the relative and dynamic characteristics of the distribution. The overall hardness and total concentration of dissolved solids and other characteristic pollutants exceeded the threshold value for grade IV groundwater quality standard in the southern area (monitoring wells S1 and S3), in agreement with the optimized model assessment. Thus we have verified the reliability of the optimized model.

Key words: groundwater, evaluation of water quality, fuzzy synthetic evaluation, relative membership degree, classification criteria